Discovery of variable VHE γ-ray emission from the binary system 1FGL J1018.6–5856

¹ Universität Hamburg, Institut für Experimentalphysik, Luruper Chaussee 149, 22761 Hamburg, Germany
² Max-Planck-Institut für Kernphysik, PO Box 103980, 69029 Heidelberg, Germany
³ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁴ National Academy of Sciences of the Republic of Armenia, Marshall Baghramian Avenue, 24, 0019 Yerevan, Armenia
⁵ Yerevan Physics Institute, 2 Alikhanian Brothers St., 375036 Yerevan, Armenia
⁶ Institut für Physik, Humboldt-Universität zu Berlin, Newtonstr. 15, 12489 Berlin, Germany
⁷ University of Namibia, Department of Physics, Private Bag 13301, Windhoek, Namibia
⁸ GRAPPA, Anton Pannekoek Institute for Astronomy, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁹ Department of Physics and Electrical Engineering, Linnaeus University, 351 95 Växjö, Sweden
¹⁰ Institut für Theoretische Physik, Lehrstuhl IV: Weltraum und Astrophysik, Ruhr-Universität Bochum, 44780 Bochum, Germany
¹¹ GRAPPA, Anton Pannekoek Institute for Astronomy and Institute of High-Energy Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
¹² Institut für Astro- und Teilchenphysik, Leopold-Franzens-Universität Innsbruck, 6020 Innsbruck, Austria
¹³ School of Chemistry & Physics, University of Adelaide, 5005 Adelaide, Australia
¹⁴ Centre for Space Research, North-West University, 2520 Potchefstroom, South Africa
¹⁵ LUTH, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹⁶ LPNHE, Université Pierre et Marie Curie Paris 6, Université Denis Diderot Paris 7, CNRS/IN2P3, 4 place Jussieu, 75252 Paris Cedex 5, France
¹⁷ Laboratoire Univers et Particules de Montpellier, Université Montpellier 2, CNRS/IN2P3, CC 72, place Eugène Bataillon, 34095 Montpellier Cedex 5, France
¹⁸ DSM/Irfu, CEA Saclay, 91191 Gif-Sur-Yvette Cedex, France
¹⁹ Astronomical Observatory, The University of Warsaw, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
²⁰ Aix-Marseille Université, CNRS/IN2P3, CPPM UMR 7346, 13288 Marseille, France
²¹ Instytut Fizyki Jądrowej PAN, ul. Radzikowskiego 152, 31-342 Kraków, Poland
²² Funded by EU FP7 Marie Curie, grant agreement No. PIEF-GA-2012-332350,  
²³ School of Physics, University of the Witwatersrand, 1 Jan Smuts Avenue, Braamfontein, 2050 Johannesburg, South Africa
²⁴ Landessternwarte, Universität Heidelberg, Königstuhl, 69117 Heidelberg, Germany
²⁵ Oskar Klein Centre, Department of Physics, Stockholm University, Albanova University Center, 10691 Stockholm, Sweden
²⁶ Institut für Astronomie und Astrophysik, Universität Tübingen, Sand 1, 72076 Tübingen, Germany
²⁷ Laboratoire Leprince-Ringuet, École Polytechnique, CNRS/IN2P3, 91128 Palaiseau, France
²⁸ APC, AstroParticule et Cosmologie, Université Paris Diderot, CNRS/IN2P3, CEA/Irfu, Observatoire de Paris, Sorbonne Paris Cité, 10 rue Alice Domon et Léonie Duquet, 75205 Paris Cedex 13, France
²⁹ Univ. Grenoble Alpes, IPAG, 38000 Grenoble, France CNRS, IPAG, 38000 Grenoble, France
³⁰ Department of Physics and Astronomy, The University of Leicester, University Road, Leicester, LE1 7RH, UK
³¹ Nicolaus Copernicus Astronomical Center, ul. Bartycka 18, 00-716 Warsaw, Poland
³² Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Strasse 24/25, 14476 Potsdam, Germany
³³ Laboratoire d’Annecy-le-Vieux de Physique des Particules, Université Savoie Mont-Blanc, CNRS/IN2P3, 74941 Annecy-le-Vieux, France
³⁴ DESY, 15738 Zeuthen, Germany
³⁵ Obserwatorium Astronomiczne, Uniwersytet Jagielloński, ul. Orla 171, 30-244 Kraków, Poland
³⁶ Université Bordeaux, CNRS/IN2P3, Centre d’Études Nucléaires de Bordeaux Gradignan, 33175 Gradignan, France
³⁷ Universität Erlangen-Nürnberg, Physikalisches Institut, Erwin-Rommel-Str. 1, 91058 Erlangen, Germany
³⁸ Centre for Astronomy, Faculty of Physics, Astronomy and Informatics, Nicolaus Copernicus University, Grudziadzka 5, 87-100 Torun, Poland
³⁹ Department of Physics, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
⁴⁰ Heisenberg Fellow (DFG), ITA Universität Heidelberg, Germany
⁴¹ GRAPPA, Institute of High-Energy Physics, University of Amsterdam, Science Park 904, 1098 XH Amsterdam, The Netherlands
⁴² Now at Institut de Ciències de l’Espai (IEEC-CSIC), Campus UAB, Fac. de Ciències, Torre C5, parell, 2a planta, 08193 Barcelona, Spain

Received: 20 January 2015
Accepted: 9 March 2015

Abstract

Re-observations with the HESS telescope array of the very high-energy (VHE) source HESS J1018–589 A that is coincident with the Fermi-LAT γ-ray binary 1FGL J1018.6–5856 have resulted in a source detection significance of more than 9σ and the detection of variability (χ²/ν of 238.3/155) in the emitted γ-ray flux. This variability confirms the association of HESS J1018–589 A with the high-energy γ-ray binary detected by Fermi-LAT and also confirms the point-like source as a new VHE binary system. The spectrum of HESS J1018–589 A is best fit with a power-law function with photon index Γ = 2.20 ± 0.14_stat ± 0.2_sys. Emission is detected up to ~20 TeV. The mean differential flux level is (2.9 ± 0.4) × 10^-13 TeV^-1 cm^-2 s^-1 at 1 TeV, equivalent to ~1% of the flux from the Crab Nebula at the same energy. Variability is clearly detected in the night-by-night light curve. When folded on the orbital period of 16.58 days, the rebinned light curve peaks in phase with the observed X-ray and high-energy phaseograms. The fit of the HESS phaseogram to a constant flux provides evidence of periodicity at the level of N_σ> 3σ. The shape of the VHE phaseogram and measured spectrum suggest a low-inclination, low-eccentricity system with amodest impact from VHE γ-ray absorption due to pair production (τ ≲ 1 at 300 GeV).